The present invention relates to sensors, whose electrodes made of a composite material comprising oxide and platinum withstand an aggressive medium.
In screen printing technology, conductor traces 100 μm wide having a thickness of 10 to 100 μm are generated by the application of paste. Conductor traces that are 8 μm thick and 80 μm wide are also obtainable with this thick-film technology. However, with screen printing conductor traces having a thickness of less than 20 μm fray more at their borders, the thinner the application is. At 8 μm film thickness, the frayed border region has a width of about 30 μm. The finer structures of more complex thin-film or resinate technology are not resistant to aggressive media.
Impedance sensors, particularly based on an electrode comb structure, contain at least two mutually non-contacting platinum electrodes on ceramic substrates, for example on Al2O3 substrates. In contrast to heating resistors or measuring resistors, as for example temperature sensors, the two electrodes are each connected only to one potential of an electric power source, so that a voltage can be applied between the electrodes. A change of the dielectric between the electrodes is measured. The electrodes are brought into contact with the medium to be measured and are therefore arranged on an external side of a chip. If the electrodes, as for example soot sensors, are exposed to an aggressive medium, only electrodes produced by thick-film technology are suitable. However, such electrodes are inaccurate compared to less resistant sensors producible in thin-film or resinate technology.